1. Field of the Invention:
The present invention relates to an optical amplifier for use in a long-haul lightwave communication system, and more specifically to an optical amplifier module capable of direct amplification of a signal light with the use of a semiconductor laser.
2. Description of the Prior Art:
Recent rapid progress in the telecommunication field requires more and more improved transmission technology capable, for example of transmitting a big volume of information to further remote sites. To satisfy such requirements a fiber optic transmission system has been proposed hitherto. An optical fiber, a low loss medium enjoys long-distance transmission. To extend the transmission distance, the fiber optic transmission sYstem includes an optical repeater inserted into an optical fiber transmission line. For such an optical repeater, a system is known until now which converts an optical signal to an electrical one and after some electrical processings renders the resulting electric signal an electro-optic conversion to reproduce an optical signal. Additionally, recent development in this field find another example of such an optical repeater capable of directly amplifying an optical signal without conversion of the signal to an electric one. The latter optical repeater has an advantage in being miniaturized and simplified, compared with the former optical repeater to effect the electro-optic conversion. Such an optical amplifier may include a semiconductor laser, for example. An example of an optical fiber with a semiconductor laser is disclosed in "Low Loss Coupling between Semiconductor Lasers and Single-mode Fiber Using Tapered Lensed Fibers" British Telecom. Technol J. Vol. 4, No. 2, Apr. 1986. FIG. 1 illustrates the configuration, where a semiconductor laser amplifier chip 21 is directly coupled with tapered fibers 22. FIG. 2 illustrates the configuration of the optical amplifier of FIG. 1 being disposed in a fiber optical transmission system. Such a configuration is disclosed in "400 Mbit/s, 372 km Coherent Transmission Experiment Using In-Line Optical Amplifiers", Electronics Letters Vol. 24, No. 1, Jan. 1988. That is, the optical amplifier device in a practical application is comprised of an optical amplifier 31, an isolator 32, a filter 33, a coupling 34, and an output power monitor 35, all members connected in series, and further connected with an optical fiber 36 and an optical coupler 37.
However, the conventional optical amplifier devices of the type described above suffer from the following difficulties: The conventional optical amplifier coupled with the semiconductor laser as illustrated in FIG. 1 is unstable in optical coupling between the optical fibers and the semiconductor laser because the optical coupling is done directly. The optical amplifier device of the type as illustrated in FIG. 2 has drawbacks because the alignment among constituent optical members is difficult.